Low-cost solar cell metallization over tco and methods of their fabrication

ABSTRACT

Methods for fabricating busbar and finger metallization over TCO are disclosed. Rather than using expensive and relatively resistive silver paste, a high conductivity and relatively low cost copper is used. Methods for enabling the use of copper as busbar and fingers over a TCO are disclosed, providing good adhesion while preventing migration of the copper into the TCO. Also, provisions are made for easy soldering contacts to the copper busbars.

BACKGROUND

1. Field of the Invention

The subject invention relates to solar photovoltaic cells and, morespecifically, to method for manufacturing low cost metallization layersfor such cells and the resulting cell device structure.

2. Related Art

The silkscreen, silver-paste metallization technology has been developedmainly by the “traditional” diffused junction solar cell manufacturers.In such cells, a top layer of silicon nitride is used and then themetallization is formed on top of the silicon nitride. However,electrical contact must be made between the metallization and theemitter—through the silicon nitride layer. Therefore, silkscreen is usedto deposit the silver paste and then the cell is annealed at hightemperature (e.g., 950° C.) so that the silver paste diffuses throughthe silicon nitride layer and makes contact to the emitter. Sincediffused-junction solar cells make the bulk of the solar market (by avery large margin), silver-paste technology became a de facto standardin the solar cell industry and much of the manufacturing and developmentefforts are directed at improving the conductivity of the resultingmetallization using silver paste.

A specific metallization layer that is particularly relevant to thesubject invention is busbar and fingers over a transparent conductiveoxide (TCO). One solar cell architecture that incorporates a silverbusbar and fingers over TCO is known as the HIT cell, available fromSanyo® of Japan. FIG. 1 illustrates the general structure of the HITmodule. A high quality (Czochralski grown) single crystal silicon waferof n-type is used as the substrate 100. The substrate 100 is about 200micron thick and a square of about 125 mm by 125 mm. The substratesurfaces are texturized to form pyramid shapes throughout the surface,but this is not shown in FIG. 1 due to the minute size of the pyramids.The top surface is coated with a thin layer of an amorphous intrinsicsilicon layer 105. A thin layer 110 of amorphous p-type silicon isdeposited over the intrinsic layer 105. A layer of TCO 115, e.g., ZnO₂,ITO (Indium Tin Oxide), or InSnO, is deposited over the p-type layer.Then, busbar 120 and fingers 125 are fabricated over the TCO, generallyby silk screen followed with anneal. The same is done on the bottomsurface, wherein an i-layer 130, p-layer 135, and TCO layer 140 aredeposited on the bottom surface, followed by busbar 145 and fingers (notvisible in FIG. 1). The HIT cell, while being of relatively highefficiency (currently available at about 20% efficiency) is veryexpensive to manufacture. While part of the cost being the high gradesilicon substrate used, other part of the cost is the high cost of thesilver paste-based busbar and fingers. Additionally, the silverpaste-based busbar and fingers pose a reliability problem in that theytend to delaminate with time.

FIG. 2 illustrate another structure, known as SmartSilicon®, andavailable from Sunpreme of Sunnyvale, Calif. A rather “dirty”metallurgical grade silicon (MG silicon) is used for fabricatingsubstrate 200, using casting and solidification technique. Metallurgicalgrade silicon is generally of 3-5 “nines” purity, meaning 99.9%-99.999%pure, compared to Czochralski grown substrates, which are of nine-ninespurity and even higher. Metallurgical grade silicon is generally used inthe manufacture of aluminium-silicon alloys to produce cast parts,mainly for the automotive industry. It is also added to molten cast ironas ferrosilicon or silicocalcium alloys to improve its performance incasting thin sections, and to prevent the formation of cementite at thesurface. MG silicon has been thought to be useless for semiconductor andsolar applications. See, e.g., Towards Solar Grade Silicon: Challengesand Benefits for Low Cost Photovoltaics, Sergio Pazzini, Solar EnergyMaterials & Solar Cells, 94 (2010) 1528-1533 (“As shown before, MG gradesilicon is much too dirty to be employed for EG and PV applications.”),and Solar Energy website of the U.S. Department of Energy: “to be usefulas a semiconductor material in solar cells, silicon must be refined to apurity of 99.9999%.” (Available athttp://www1.eere.energy.gov/solar/silicon.html.) This is generallyreferred to as 6N, or solar grade silicon, SoG Si. Therefore, effortshave been made to produce what is referred to as “upgraded”metallurgical silicon (UMG silicon). However, to date, these effortshave not shown great success and come at high cost, especially for thehigh energy required for the “upgrade” process. Conversely, Sunpreme hasshown that by using p-type doped MG silicon substrate and formingspecific layers of amorphous silicon, a relatively cheap solar cell canbe formed that has conversion efficiency higher than that ofconventional thin-film solar cells.

The SmartSilicon solar cell is formed using a p-type MG siliconsubstrate 200, forming an amorphous intrinsic layer 205 on the topsurface, forming an amorphous n-type layer 210 over the intrinsic layer205, forming a TCO 215 over the n-layer. A back metallization layer isformed by depositing a titanium layer 230 over the entire back surfaceof the substrate 200, and depositing a layer of aluminum 235 over thetitanium layer 230. The busbar 220 and fingers 225 are formed of silver,using the silk screen method. The SmartSilicon cell's attractiveness isin its conversion efficiency being competitive with that of pure siliconsolar cells, while using an extremely cheap MG silicon substrate.Consequently, the relative cost contribution of the silver metallizationprocess to the cost of the entire cell increases.

While the solar industry embraces the silkscreen silver pastemetallization process, the subject inventors have determined that thereis a need to provide a cost-effective solution for busbar and fingermetallization over a TCO, that is cheaper and more reliable thansilkscreen silver paste, and that has lower resistivity than silverpaste.

SUMMARY

The following summary of the invention is included in order to provide abasic understanding of some aspects and features of the invention. Thissummary is not an extensive overview of the invention and as such it isnot intended to particularly identify key or critical elements of theinvention or to delineate the scope of the invention. Its sole purposeis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented below.

Various embodiments of the subject invention provide methods forfabricating busbar and finger metallization over or together with a TCO.Rather than using expensive and relatively resistive silver paste,consisting of pure silver and glass frit with binders, with aresistivity of many times more than that for bulk Silver, embodiments ofthe invention utilize the high conductivity and relatively low costcopper (or alloys containing copper) deposited at room temperature, witha resistivity of around 2 μOhmcm. Various embodiments provide methodsfor enabling the use of copper as busbar and fingers over a TCO,providing good adhesion while preventing migration of the copper intothe TCO. Also, provisions are made for easy soldering contacts to thecopper busbars. The various embodiments of the invention can be appliedto any solar cell structure that utilizes TCO, such as the HIT or theSmartSilicon. While embodiments of the invention are particularlybeneficial for solar cells made of MG silicon, they can be applied tosolar cells made on any substrate.

According to a further aspect of the invention, a solar cell isprovided, comprising: a substrate having a back surface and a frontsurface, the front surface designed for facing the sun; a photovoltaicstructure formed on the front surface; an ITO formed over thephotovoltaic structure; and, front contacts formed over the ITO, thefront contacts comprising busbars and fingers comprising copper. Insteadof copper, an alloy comprising copper and other materials, such as,e.g., nickel and/or tin can be substituted.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and features of the invention would be apparent from thedetailed description, which is made with reference to the followingdrawings. It should be appreciated that the detailed description and thedrawings provides various non-limiting examples of various embodimentsof the invention, which is defined by the appended claims.

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify the embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

FIG. 1 illustrates a HIT solar cell according to the prior art.

FIG. 2 illustrates a SmartSilicon® solar cell according to the priorart.

FIG. 3 illustrates a process according to an embodiment of theinvention.

FIG. 4 illustrates the resulting structure of the process of FIG. 3.

FIG. 5 illustrates another process according to an embodiment of theinvention.

FIG. 6 is a flow chart of another process according to an embodiment ofthe invention, while

FIG. 7 illustrates a cross section of the progression of the resultingstructure.

FIG. 8 is a flow chart of another process according to an embodiment ofthe invention, while

FIG. 9 illustrates a cross section of the progression of the resultingstructure.

FIG. 10 illustrates another embodiment of the invention, while

FIG. 11 illustrates a cross section of the progression of the resultingstructure.

FIG. 12 illustrates an embodiment of the invention with metallizationbefore TCO process, while

FIG. 13 illustrates a cross section of the progression of the resultingstructure.

FIG. 14 illustrates another embodiment of the invention withmetallization before TCO process, while

FIG. 15 illustrates a cross section of the progression of the resultingstructure.

FIG. 16 illustrates yet another embodiment of the invention withmetallization before TCO process, while

FIG. 17 illustrates a cross section of the progression of the resultingstructure.

DETAILED DESCRIPTION

Embodiments of the subject invention provide methods for manufacturingsolar cells at reduced costs. Embodiments of the invention provide alower cost alternative to silver metallization, which provide lowresistivity—nearly ten times lower resistivity than silver paste basedmetallization—thereby enhancing current collection from the photovoltaiccell. Also, lower deposition temperatures of essentially roomtemperature in order to achieve the lower resistivities. Various methodsare provided to increase adhesion and enable soldering to the busbars.

FIG. 3 is a flowchart illustrating a process according to an embodimentof the invention, while FIG. 4 illustrates a cross-section of theresulting structure, exemplified by the broken-line arrow in FIGS. 1 and2. In FIG. 3, the process starts after the photovoltaic (PV) cell 400has been fabricated on a substrate and a TCO layer 405, e.g., an ITO hasbeen deposited over the PV cell 400. The PV cell 400 may be any PV cellrequiring a TCO as its top layer, for example, the HIT structure fromSanyo or the SmartSilicon structure from Sunpreme. In step 300, a mask410 is formed over the TCO to delineate the design of the metallization.The mask may be, for example, a hot wax deposited using technique suchas inkjet printing, or a resist material that can be cured via exposureto heat or certain illumination, e.g., UV curing, and being depositedusing, e.g., the silkscreen technique. According to one embodiment, aninkjet system is used to deposit hot wax mask. Thereafter, the wax maskis reflowed by annealing the wafer, so as to provide enhanced coverage,especially when the surface of the wafer is textured.

Then, in step 305 a barrier/adhesion layer 415 is deposited. This layeris needed for two reasons. First, it is difficult to have copper adhereto TCO, especially to ITO. Second, copper tends to migrate and a studyalready showed that ITO is not a very good diffusion barrier to copper.The adhesion/barrier layer may be of a transition metal such as, e.g.,chromium, nickel, titanium, etc. It may be deposited by, e.g.,electroplating, electroless plating, PVD sputtering, etc. In step 310copper layer 420 is plated over the barrier/adhesion layer 415. In thisexample the copper is plated using electroplating. In step 315 a caplayer 425 is formed over the copper 420. The cap layer 425 may beelectroplated tin layer, which enables easy soldering onto themetallization layer, so as to connect a plurality of solar cellstogether, normally in a series connection. Alternatively, the barrierlayer 415 may be sensitized by dipping in a liquid solution containingPd++ (e.g. PdC12) and then electrolessly plated with Copper. The finalNi layer is also electroless plated on top of the electroless Copper.Both electroless plating steps do not require an external field to beapplied during the plating process.

FIG. 5 illustrates another process according to an embodiment of theinvention. The process of FIG. 5 is similar to that of FIG. 3, exceptthat a plasma treatment step has been added to the process flow.According to one embodiment, as shown in FIG. 5, the plasma treatment isperformed after forming the mask 410. On the other hand, the plasmatreatment may also be performed prior to forming the mask 410. In oneembodiment the plasma treatment comprises a CH4 plasma formed in situ orusing remote plasma source. The plasma treatment may be performed atelevated temperature, e.g. 100° C.-200° C. for, say, 5-20 minutes. Theplasma treatment helps forming good adhesion to the TCO, especially whenusing electroplating to form the barrier/adhesion layer 415. The plasmatreatment is used to reduce the TCO to enable better electroplating. Inother embodiments, CH4/Ar or Ar/H2 plasma is used. According to yetanother embodiment, an H2 plasma treatment is performed before the maskis formed.

FIG. 6 is a flow chart of another process according to an embodiment ofthe invention, while FIG. 7 illustrates a cross section of theprogression of the resulting structure. As before, a PV solar cell 700having a top TCO layer 705 is prepared for front surface metallization,i.e., fingers and busbars. The PV cell 700 is placed inside a PVDchamber having a target for a barrier/adhesion layer, e.g., nickel ortitanium. For an improved adhesion, it is suggested that for this step aPVD chamber having a sputtering shutter be used. In step 600, theshutter is closed and plasma is ignited so as to treat the target bysputtering the target with plasma while the shutter is closed, so thatno sputtered material reaches the PV cell. This can be a very shortprocess, e.g., 2-10 minutes. At step 605 the shutter is opened and theplasma is maintained, so that an adhesion/barrier layer 710 is depositedon the TCO layer 705. The barrier layer can be 250-750 Å thick.

Step 610 is optional, but is shown in FIG. 6 in broken-line box and inFIG. 7 as part of the device for the completeness of illustration. Instep 610 a seed copper layer 715 is deposited over the barrier/adhesionlayer 605. In this embodiment the seed layer 715 is also PVD sputteredand can be very thin, e.g., 100-500 ÅA.

In step 615 a mask 720 is formed, e.g., using wax inkjet printing orphotoresist silkscreen, so as to delineate the metallization design. ThePV cell is then transferred to an electroplating system to electroplatecopper layer 725. In step 625 a cap layer 730 is also electroplated overthe copper layer 725. Here, the thickness of the cap layer is depositedthicker than the final desired thickness since, as will be shown later,part of this cap layer 730 will be removed during further processing. Asbefore, the cap layer may be nickel, chromium, tin, etc. In step 630 themask 720 is removed using proper solvent, depending on the type of maskmaterial used. For example, a diluted mixture of KOH (less than 10%) canbe used to remove wax or resist mask at room or elevated temperature(e.g., 50° C.). Then, a mixture of sodium persulfate or ammoniumpersulfate is used to remove the copper that was exposed when the maskwas removed. Thereafter, a mixture of potassium permanganate is used toremove the part of the barrier/adhesion layer that was exposed by theremoval of the copper. In this step, part of the cap layer may also beremoved, which is why it is suggested to make the cap layer thicker thanthe desired final thickness. Also, in this step the potassiumpermanganate does not etch the TCO, so that in effect there is a naturaletch stop when the barrier/adhesion layer is fully removed. In order toprevent any lateral etching of the barrier and seed layers, especiallyundercutting of the barrier layer underneath the copper fingers, thepermanganate etching may be done with a jet spray to impartdirectionality to the etch, minimize isotropic etching resulting fromimmersing the wafer in a stationary liquid bath.

FIG. 8 is a flow chart of another process according to an embodiment ofthe invention, while FIG. 9 illustrates a cross section of theprogression of the resulting structure. This embodiment utilizesconventional silkscreen technology together with plating technology. Instep 800, a silkscreen system is used to deposit a barrier/adhesionlayer 910 using, e.g., chromium paste, titanium paste, or standardsilver paste. The deposited barrier/adhesion layer 910 is annealed atrelatively low temperatures, such as 150°-250° C. In optional step 805 aseed layer 915 made of copper paste is deposited, also using silkscreentechnique. Then, a mask 920 is formed in step 810, to delineate thedesign of the busbar and fingers. In this example, since thebarrier/adhesion and seed layers were formed using silkscreentechnology, it may be simpler to use silkscreen to also deposit aphotoresist mask. At step 815 the wafer is taken to an electroplatingsystem and electroplated with copper so as to form copper metallizationlayer 925. Thereafter in step 820 a cap layer 930 is deposited, alsousing electroplating. The mask 920 is then removed in step 825, so as toleave the metallization stack according to the mask design.

FIG. 10 illustrates another embodiment of the invention, while FIG. 11illustrates a cross section of the progression of the resultingstructure. In the embodiment of FIGS. 10 and 11, the process beginsbefore the deposition of the TCO. As shown in FIGS. 10 and 11, in step1000 a barrier/adhesion layer is first deposited over the top layer ofthe PV device. For example, in the HIT structure it will be depositedover the top p-type amorphous silicon layer 110, while in theSmartSilicon device it will be deposited over the top p-type amorphoussilicon layer 210. This is illustrated in FIG. 11 as adhesion layer 1110deposited over device layer 1100. In this embodiment, the adhesion layeris PVD sputtered metal, such as, e.g., titanium, tantalum, etc. Inoptional step 1005 a copper seed layer 1115 is PVD sputtered over theadhesion layer 1110. In step 1010 a mask 1120 is formed to delineate thedesign of the metallization. The mask may be inkjet wax, silkscreenphotoresist, etc. In step 1015 the wafer is electroplated with copper,to form copper metallization 1125. In optional step 1020 a cap layer1130 is electroplated over the copper layer 1125. In step 1025 the maskis removed and in step 1030 the seed and adhesion layers are removed aswell. Note that in this step since the adhesion and seed layers are muchthinner than the copper metallization layer 1125, it is very easy toetch them without harming the metallization layer 1125. Thus, the seedand adhesion layer can be removed in a diluted HF solution. Again, theuse of directional wet etching using a jet spray will help prevent anyundercutting. In step 1135 TCO is deposited over the entire substrate,thereby providing TCO over the top device layer 110, and also covering,and thereby protecting, the sidewall and the top of metallization 1125.Since the TCO provides protection over the metallization layer, step1020 of depositing a cap layer may be dispensed with. Still, it isrecommended to deposit a cap layer of easily solderable material, suchas tin. This will enable easy soldering of the PV cell array. In thisrespect, it is noted that the TCO will need to be partially removed fromthe top of the busbars for soldering. This can be easily done withdiluted HF.

For all of the above embodiments, when using electroplating, it isbeneficial to prepare the surface of the TCO so that it is hydrophilic.This can be done by any of the following exemplary methods, or anycombination thereof. According to one embodiment, the wafer with the ITOis rinsed in a “soap-like” solution to clean the surface of the TCO fromany organic material. An example of such a solution may be the Micro-90,commercially available from Cole-Parmer of Vernon Hills, Ill. Accordingto another embodiment, the surface of the TCO is treated with asurfactant. The surfactant treatment may be instead or in addition tothe cleaning step. An example of surfactant may be a solution of sodiumalkyl sulfates, mainly the lauryl, such as sodium dodecyl sulfate.According to another embodiment, the TCO is treated with UV light in anozone atmosphere. This can be done at room or elevated temperature,e.g., 100°-200° C.

FIG. 12 illustrates an embodiment of the invention with metallizationbefore TCO process, while FIG. 13 illustrates a cross section of theprogression of the resulting structure. According to this embodiment,the metallization is fabricated first directly on the top layer 1300 ofthe photovoltaic device, and the TCO is fabricated later. In step 1200 amask 1320 (FIG. 13) is formed over the top layer of the photovoltaicdevice to delineate the fingers and busbars. The top layer may be, e.g.,the amorphous n-type or p-type silicon layer of the photovoltaic devicejunction. In step 1205 a barrier/adhesion layer 1310 is fabricateddirectly over the top device layer 1300, also somewhat covering the mask1320. Optionally, in step 1210 a seed layer 1315 is fabricated over thebarrier/adhesion layer. Then, in step 1215 the mask is replaced by firstremoving the mask 1320 so as to leave only metallization trace 1325, andthen forming a new mask 1322. Then is step 1220 copper layer 1330 isformed by, e.g., electroplating. It should be noted that ifelectroplating is used, copper will be formed only where electricalpotential is exposed, such that no copper will be formed over the mask.In step 1225 a cap layer 1335 is formed over the copper. Again, ifelectroplating is used, the cap layer will be formed only over thecopper and not over the mask. Then is step 1230 the mask is removed, soas to leave only the metallization structure 1345. In step 1235 TCO isdeposited over the entire wafer, thereby forming somewhat of aninterdigit structure of the metallization a TCO over the top layer ofthe photovoltaic device. In this respect, it is possible to select thematerial of the cap layer such that TCO will not adhere to it.Regardless, the coverage of the stack 1345 by TCO is not detrimentaland, in fact can be used as a good protection layer against oxidation ofthe stack.

FIG. 14 illustrates another embodiment of the invention withmetallization before TCO process, while FIG. 15 illustrates a crosssection of the progression of the resulting structure. According to thisembodiment, the metallization is fabricated first directly on the toplayer 1500 of the photovoltaic device, and the TCO is fabricated later.In step 1400 a mask 1520 (FIG. 15) is formed over the top layer 1500 ofthe photovoltaic device to delineate the fingers and busbars. The toplayer 1500 may be, e.g., the amorphous n-type or p-type silicon layer ofthe photovoltaic device junction. In step 1405 a barrier/adhesion layer1505 is fabricated directly over the top device layer 1500, alsosomewhat covering the mask 1520. Optionally, in step 1410 a seed layer1510 is fabricated over the barrier/adhesion layer. Then, in step 1420copper layer 1515 is formed by, e.g., electroplating. In step 1425 a caplayer 1520 is formed over the copper. Then is step 1430 the mask isremoved, so as to leave only the metallization structure 1525. In step1435 TCO is deposited over the entire device. In this respect, it ispossible to select the material of the cap layer such that TCO will notadhere to it. Regardless, the coverage of the stack 1525 by TCO is notdetrimental and, in fact can be used as a good protection layer againstoxidation of the stack.

FIG. 16 illustrates yet another embodiment of the invention withmetallization before TCO process, while FIG. 17 illustrates a crosssection of the progression of the resulting structure. This embodimentutilizes conventional silkscreen technology together with platingtechnology, whereby the metallization is fabricated first directly onthe top layer 1700 of the photovoltaic device, and the TCO is fabricatedlater. In step 1700, a silkscreen system is used to deposit abarrier/adhesion layer 1705 directly on the top device layer 1700,using, e.g., chromium paste, titanium paste, or standard silver paste.The deposited barrier/adhesion layer 1705 is annealed at relatively lowtemperatures, such as 150°-250° C. In optional step 1605 a seed layer1710 made of copper paste is deposited, also using silkscreen technique.Then, a mask 1720 is formed in step 1610, to delineate the design of thebusbar and fingers. In this example, since the barrier/adhesion and seedlayers were formed using silkscreen technology, it may be simpler to usesilkscreen to also deposit a photoresist mask. At step 1615 the wafer istaken to an electroplating system and electroplated with copper so as toform copper metallization layer 1725. Thereafter in step 1620 a caplayer 1730 is deposited, also using electroplating. The mask 1720 isthen removed in step 1625, so as to leave the metallization stack 1745according to the mask design. Then, in step 1630 a TCO layer 1705 isdeposited over the entire device.

It should be understood that processes and techniques described hereinare not inherently related to any particular apparatus and may beimplemented by any suitable combination of components. Further, varioustypes of general purpose devices may be used in accordance with theteachings described herein. It may also prove advantageous to constructspecialized apparatus to perform the method steps described herein.

The present invention has been described in relation to particularexamples, which are intended in all respects to be illustrative ratherthan restrictive. Those skilled in the art will appreciate that manydifferent combinations of hardware, software, and firmware will besuitable for practicing the present invention. Moreover, otherimplementations of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims. For example,the disclosure relates to using copper; however, it should beappreciated that an alloy comprising copper and other materials, suchas, e.g., nickel and/or tin can be substituted.

1. A solar cell, comprising: a substrate having a back surface and afront surface, the front surface designed for facing the sun; aphotovoltaic structure formed on the front surface; a transparentconductive oxide (TCO) layer formed over the photovoltaic structure;and, front metallization layer formed over the TCO, the frontmetallization layer comprising a copper layer.
 2. The solar cell ofclaim 1, wherein the TCO comprises an indium tin oxide (ITO).
 3. Thesolar cell of claim 1, wherein the front metallization layer furthercomprises an adhesion/barrier layer formed between the TCO and thecopper layer.
 4. The solar cell of claim 3, wherein the frontmetallization layer further comprises a cap layer formed over the copperlayer.
 5. The solar cell of claim 3, wherein the front metallizationlayer further comprises a seed copper layer formed between theadhesion/barrier layer and the copper layer.
 6. The solar cell of claim3, wherein the adhesion/barrier layer comprises one of nickel, chromium,or titanium or alloy thereof.
 7. The solar cell of claim 6, wherein theadhesion/barrier layer comprises one of electroplated nickel orelectroplated chromium.
 8. The solar cell of claim 6, wherein theadhesion/barrier layer comprises one of sputtered nickel or sputteredtitanium.
 9. The solar cell of claim 1, wherein the copper layercomprises electroplated copper or an alloy comprising copper.
 10. Thesolar cell of claim 4, wherein the cap layer comprises tin.
 11. Thesolar cell of claim 1, wherein the photovoltaic structure comprises adoped amorphous silicon layer.
 12. The solar cell of claim 11, whereinthe photovoltaic structure further comprises an intrinsic amorphoussilicon layer between the substrate and the doped amorphous silicon. 13.The solar cell of claim 11, wherein the substrate comprises a dopedsilicon, and wherein the doped amorphous silicon layer is doped oppositethe doping of the substrate.
 14. A method for fabricating a solar cell,comprising: fabricating a photovoltaic structure over a substrate;fabricating a transparent conductive oxide (TCO) layer over thephotovoltaic structure; and, fabricating front metallization layer inelectrical contact with the TCO, the front metallization layercomprising a copper layer or an alloy with 50% or more of copper. 15.The method of claim 14, wherein fabricating front metallization layercomprises electroplating the copper layer.
 16. The method of claim 15,further comprising treating the TCO to become hydrophilic prior tofabricating the metallization layer.
 17. The method of claim 16, whereinfabricating front metallization layer further comprises fabricating abarrier/adhesion layer prior to electroplating the copper layer.
 18. Themethod of claim 17, wherein fabricating front metallization layerfurther comprises fabricating a cap layer over the copper layer.
 19. Themethod of claim 17, wherein fabricating a barrier/adhesion layercomprises sputtering a metal layer.
 20. The method of claim 19, whereinfabricating front metallization layer further comprises forming a maskto delineate the metallization design.
 21. The method of claim 14,wherein fabricating front metallization layer comprises electroplatingthe metallization layer directly over the photovoltaic structure.